get rid of boilerplate switches in posix_acl.h

the only potentially subtle thing here: get_cached_acl()
is never called with the second argument other than
ACL_TYPE_{ACCESS,DEFAULT}.  IOW, that return ERR_PTR(-EINVAL)
in there might as well be BUG().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>